On August 12, 1999, at 10:40 a.m., an unintentional detonation of explosives occurred at the Trapper Mine, Derringer Pit, causing fatal injuries to Joseph T. Koonce, blaster helper. The accident occurred during the priming process when an assembled slider-primer was unintentionally released and slid down the low strength detonating cord into the 102-foot deep, bottom primed hole. At the time of the detonation, the victim was attempting to intercept and recover the fallen slider-primer, and was positioned on the drill cuttings and partially over the blast hole. Subsequent investigation identified the impact of the free falling slider-primer with the fixed suspended bottom-primer as the probable cause of the unplanned detonation.

An assembled string of 4 primers was being used in each borehole on a 60-foot wide by 300-foot long bench. To prepare a hole for blasting, all 4 primers were placed on a single piece of non-electric, 2.4 grains per foot, detonating cord (Detaline). The bottom primer was lowered to the bottom of the borehole and then raised and suspended off the bottom of the borehole approximately three feet. The detonating cord was tied to a piece of wooden lathe, which was placed in the outside portion of the drill cuttings. The remaining three primers, which were strung on the detonating cord, were placed in the drill cuttings, between the lathe and the top of the cone shaped drill cuttings. As blasting agent was loaded into the hole, the three primers were slid down the detonating cord to the proper location in the blasting column.

GENERAL INFORMATION

The Trapper Mine is a surface coal mine located 6.5 miles south of Craig, Colorado, on State Route 13, and operated by Trapper Mining Inc., a Delaware corporation.

The mine has three pits. Topsoil is removed with scrapers and is either placed directly on a previously mined area or stockpiled for later use. Overburden ranging in thickness from 20 to 150 feet, is drilled, loaded with explosives, blasted, and removed by 30 cubic yard draglines. The coal is either drilled and blasted, or ripped with a dozer and is loaded into surface haul trucks by excavators. A private road is used to transport the coal to a nearby power plant.

The mine has 158 employees and produces approximately 10,000 tons daily. The mine normally works three rotating shifts per day, seven days per week.

The last regular safety and health inspection at this mine, prior to the accident, was completed by the Mine Safety and Health Administration (MSHA) on January 15, 1999.

On Thursday, August 12, 1999, at 6:00 a.m., the blasting crew went to work under the supervision of Jim Phillips, blasting foreman. The crew blasted a prepared bench at the north end of the Derringer Pit at 7:15 a.m. Phillips then met with the blasting crew and gave orders to go to the newly drilled benches at the south end of the Derringer Pit to continue normal blasting procedures. Dale Kettle and Carl Thomas, blaster helpers, worked at assembling bottom primers and slider-primers and placing them on the bench beside the previously drilled blast holes. Joseph Koonce (victim) and Dennis Watson, blaster helpers, were in the process of priming separate blast holes averaging 100 feet deep. The priming process involved placing a bottom-primer on the Detaline and lowering the bottom-primer to the bottom of the borehole. Additional Detaline was removed from the spool and the line was cut. Three slider-primers were threaded over the top end of the cut Detaline. The bottom-primer was raised and suspended off the bottom approximately 3 feet, and the top end of the line was tied to a piece of wooden lathe which was placed in the cone-shaped drill cuttings. The weight of the three slider-primers threaded on the Detaline and placed on the outside of the drill cuttings kept the bottom-primer suspended off the bottom of the borehole.

The pre-detonation accident occurred at 10:40 a.m. Immediately before the explosion, the blaster helpers heard Mr. Koonce shout. Mr. Thomas saw Mr. Koonce positioned on the drill cuttings, over the borehole, with his left arm in the borehole. The blaster helpers later reported that they believed Mr. Koonce was trying to retrieve a slider-primer which was sliding or had slid into the hole after the knot had been placed on the wooden lathe. Joseph T. Koonce was killed instantly by the unintentional detonation and was pronounced dead at the scene.

INVESTIGATION OF THE ACCIDENT

MSHA's investigation of the accident started on August 12, 1999. A list of those persons who were interviewed and who participated in the investigation is contained in Appendix A. Interviews were conducted at the mine site on August 14, 1999.

The field investigation and interviews eliminated high borehole temperatures, lightning, and electrical currents as sources for the unintentional detonation. Those persons interviewed also indicated that on previous occasions the accidental release of slider primers and the free falling down the detonating cord onto the bottom primer had occurred without any detonations.

Examination of explosives still in the mine magazine found a few malformed primers with centerline protrusions where the detonator body cavity is formed during manufacture. The investigative team included representatives of the explosives manufacturers and MSHA Approval and Certification Center personnel who were knowledgeable about other unintentional detonations using the blasting system being investigated. Because impact detonations have rarely occurred and then only under unusual circumstances, MSHA requested the National Institute of Occupational Safety and Health (NIOSH) to conduct a series of tests to simulate various impact scenarios. The mine and explosives manufacturers cooperated fully in providing the primers and detonators in use at the time of the accident and any assistance requested.

Test results indicated that the observed malformation of the primers did not contribute to an impact detonation but did confirm that a sharp impact on a specific portion of the detonator's base would trigger an unintentional detonation.

Such unintentional detonations would occur if gravel or rock fragments were between the fixed, suspended bottom primer and the free falling slider primer at a specific location on impact. See Appendix F for the summary report of the explosive testing conducted by NIOSH.

DISCUSSION

1. The accident occurred on August 12, 1999, at 10:40 a.m., in the Derringer Pit.

2. Multiple benches, measuring 60 feet wide by 300 feet long, were being used for drill site locations.

3. Typical boreholes being drilled were 10 inch diameter by 102 feet deep, with burden and spacing of 25 feet by 30 feet.

4. The blasting system within each drilled hole consisted of the following components:

c. Detaline Delay Primer Unit to slide down the Detaline Detonating Cord for use in multiple deck charges.

d. One pound High Detonation Pressure (HDP) Boosters.

e. A Bottom-Primer, made by inserting item No. 4b into item No. 4d, for use at the bottom of each hole.

f. Slider-Primers, made by inserting item No. 4c into item No. 4d, for use at the top and bottom of each ANFO powder column.

5. In order to prepare a borehole for blasting, four one-pound primers were placed on the drill cuttings of each predrilled borehole. The priming process involved placing a bottom-primer on the Detaline and lowering the bottom-primer to the bottom of the hole. Additional Detaline was removed from the spool and the line was cut. Three slider-primers were threaded over the top end of the cut Detaline. The bottom-primer was raised and suspended off the bottom approximately 3 feet, and the top end of the line was tied to a piece of wooden lathe, which was placed in the cone-shaped drill cuttings. The weight of the three slider-primers threaded on the Detaline and placed on the outside of the drill cuttings kept the bottom-primer suspended off the bottom of the borehole.

6. Joseph T. Koonce (victim) was qualified as a Surface Shotfirer in the State of Colorado on August 7, 1993. Koonce had received ANFO Blast Truck Operator training, and Blaster/Blast Helper training, on June 7, 1999. Koonce had 18 years and 40 weeks of total mining experience, which included 7 years and 8 weeks experience as a Blaster Helper.

7. A video recording of the borehole after the accident indicated that initiation within the borehole occurred at the bottom of the hole. There were no other indicators within the borehole.

8. Lightning was not a factor. The weather was clear with a light breeze. Cirrus clouds were present and the temperature was approximately 75 degrees Fahrenheit.

9. Static electricity was not present.

10. Drill hole temperature was not a factor. The ambient temperature in the bottom of surrounding drill holes measured less than 60 degrees Fahrenheit.

CONCLUSION

The accident occurred during the priming process when an assembled slider-primer(s) unintentionally fell into the 102-foot deep, bottom primed borehole. The impact of the slider-primer(s) with a rock fragment(s) lying on a specific portion of the fixed suspended bottom-primer, at the bottom of the borehole, was the probable cause of the unplanned detonation of the fallen slider-primer(s), the bottom-primer, the Detaline, and the one or two remaining slider-primer(s) which were on the drill cuttings. At the time of detonation, the victim was attempting to recover the fallen slider-primer(s), and was positioned on the drill cuttings, over the borehole, with his left arm in the borehole.

ENFORCEMENT ACTIONS

The investigation did not reveal any violations of Title 30 Code of Federal Regulations that contributed to the cause of the accident.

A Section 103(k) Order, No. 7617351, dated August 12, 1999, was issued following the accident to ensure the safety of the miners.

A Section 107(a) Order, No. 7617689, dated August 17, 1999, was issued following the accident investigation. The body of the order follows:

"The preliminary investigation for the fatal explosive accident which occurred on August 12, 1999, at the Trapper Mine revealed the following:

An assembled string of 4 primers were being used in each drilled hole on a 60-foot wide by 300-foot long bench, in the Derringer pit. In order to prepare a hole for blasting, all four primers were placed on a single piece of non-electric det-cord. The first primer was lowered to the bottom of the hole and was then raised and suspended off the bottom approximately three feet. The det-cord was tied to a piece of wooden lathe which was placed in the outside portion of the drill cuttings. The remaining three primers which were strung on the det-cord, were placed in the drill cuttings, between the lathe and the top of the cone shaped drill cuttings.

The accident occurred during the priming process when an assembled slider-primer accidentally fell into a 102-foot deep, primed hole. The impact of the slider-primer with the fixed suspended bottom-primer, caused an unplanned detonation of the bottom-primer, the fallen slider-primer, and the two remaining slider-primers which were in the drill cuttings. At the time of detonation, the blaster helper was attempting to recover the fallen slider-primer, and was positioned on the drill cuttings.

Therefore, the practice of placing assembled primers on the det-cord and leaving them in the drill cuttings as described above, without a positive means to prevent primers from accidentally sliding into the hole, creates an imminent danger."